1. Field of the Invention
This invention relates to the field of semiconductors and, more specifically, to a method and apparatus for measuring minority carrier recombination lifetimes of semiconductor materials using radio frequency coupling techniques.
2. Description of the Prior Art
Semiconductors are a group of solid materials that are intermediate between conductors, which conduct electricity, and insulators, which do not conduct electricity. Semiconductors may be comprised of single elements or compounds of multiple elements. Silicon and germanium are well known examples of elemental semiconductor materials while indium phosphide and gallium arsenide are examples of compound semiconductors.
Electrons in outer shells of atoms in conductive materials, such as metals, are not bound to specific atoms and float freely from atom to atom, so such materials conduct electricity readily, whereas electrons in outer shells of atoms in insulator materials are tightly bound to their respective atoms so that they do not conduct electric current. The ability of a piece of semiconductor material to conduct electricity is a result of the semiconductor having negatively charged electrons and vacant electron energy statesxe2x80x94electron xe2x80x9cholesxe2x80x9dxe2x80x94that behave as though they are positively charged particles near the top of an energy band. Quantum states occupied by electrons can create charge carriers with negative charges that behave as negatively charged free particles. The positively charged holes are quantum energy states in the semiconductor material with an absence of the negatively charged free particles or electrons. When an electron and a hole combine, the net charge is zero. When an electric voltage is applied across a piece of semiconductor material, a positive end of the semiconductor and a negative end of the semiconductor material are established. The externally applied electric voltage causes electrons to travel between the atoms in the semiconductor material from the negative end of the semiconductor material to the positive end of the semiconductor material, while the xe2x80x9cholesxe2x80x9d travel conversely in the opposite direction. The electrons and holes are referred to as charge carriers because they provide the means of charge flow, or electric current. When a voltage is applied across a piece of semiconductor material, electrons drift toward the positive end of the semiconductor and the holes drift toward the negative end of the semiconductor material
The flow of electric current in a semiconductor can be described as motion by both electrons and holes. The semiconductor material may be xe2x80x9cdopedxe2x80x9d by the addition of a chemical impurity to increase the number of holes or electrons. The impurities producing electrons are called donors and the impurities producing holes are called acceptors. When chemical doping is performed, the generated particle, electron or hole, is called the majority carrier. The less populous particle is called the minority carrier.
When a sample of semiconductor material is in equilibrium, no external forces such as electric voltages, electric fields, magnetic fields, or temperature gradients are acting on the semiconductor material. When in an equilibrium condition, the semiconductor material is electrically neutral with the net positive charge equal to the net negative charge. Electrons are continually being excited by heat, light, or other energy during equilibrium, however, such that free electrons from lower energy bands are excited to higher energy conductor bands where they move randomly in the semiconductor material. This xe2x80x9cgenerationxe2x80x9d of electrons for the semiconductor bands also generates a concomitant hole for each generated electron. Simultaneously, an electron moving randomly through the semiconductor material may come into close proximity to a hole and recombine with the hole. Since the net concentrations of holes and electrons in a sample of semiconductor material at equilibrium remain constant, the rate at which electron and hole pairs are generated and the rate at which they recombine must be equal.
Any deviation from equilibrium will change the electron and hole concentrations in a semiconductor to new levels. The deviation from equilibrium can be created by, for example, applying an electric voltage across the semiconductor, directing light onto the semiconductor, or increasing the temperature of the semiconductor, which will increase the concentrations or densities of excess electrons and holes in the semiconductor, or by creating new electron-hole pairs at a rate equal to the recombination rate. The excess charge carriers generated by such energy input create additional electric current flowing through the semiconductor, while deletion of charge carriers by recombination inhibits electric current flow.
When sufficient energy from an external source or stimulus is applied to the semiconductor material to increase the generation of electrons and holes to a rate that is greater than the rate of recombination of electrons and holes, the population or density of electrons and holes increases until the semiconductor material reaches a new equilibrium point. The electric current carrying capacity of the semiconductor material is proportional to the densities of the electrons and holes. Therefore, increasing the densities of electrons and holes in the semiconductor material in the presence of a voltage will increase the electric current flowing in the semiconductor material.
After removal of the external energy source or stimulus from the semiconductor material, the rate of recombination of electrons and holes will be greater than the rate of generation of electrons and holes until equilibrium in the semiconductor material is reached. However, the semiconductor""s return to its equilibrium condition will not be instantaneous. Rather, a period of time will elapse while recombination of electrons and holes occurs before the semiconductor material reaches its original equilibrium condition. During this period of time after the external energy source or stimulus to the semiconductor material has been removed and before the semiconductor material has returned to its equilibrium condition, the excess charge carriers allow the semiconductor material to continue to conduct electricity. Thus, the longer the time period it takes a semiconductor material to return to its equilibrium condition after the externally applied energy source or stimulus is removed, the longer the semiconductor material will conduct electricity after the externally applied energy source or stimulus is removed. This important characteristic of a semiconductor is known as the semiconductor""s recombination rate or minority carrier lifetime.
Many prior art devices exist to measure a semiconductor material""s minority carrier lifetime. For example, U.S. Pat. No. 5,453,703 issued to Goldfarb and U.S. Pat. No. 5,406,214 issued to Boda et al., each disclose a method or apparatus for measuring minority carrier lifetimes of semiconductor materials. Goldfarb uses a capacitance-coupling technique to measure minority-carrier recombination velocity on the surface of semiconductor materials. Unfortunately, Goldfarb""s disclosed method is not suitable for testing an entire sample of semiconductor material (i.e., a bulk sample). Boda et al. measure the microwave energy reflected from holes and electrons in the specimen under test to determine minority carrier lifetimes in semiconductors. Unfortunately, the use of reflected microwaves to measure minority carrier lifetimes creates inherent limitations, such as being limited to high resistivity samples that prevent the lifetime measurement for highly conducting materials, that limit the use of the disclosed method and apparatus. Other prior art devices and methods suffer from the problem that the output of system is not reliably linear. Therefore, these prior art devices were not consistently accurate when testing samples of different shapes, sizes, and properties. Furthermore, these prior art devices did not always display a high sensitivity. Therefore, they could not always produce a measurable output signal from which the minority carrier lifetime of the sample being tested could be determined. Thus, despite the state of the art, there remains a need for a nondestructive, contact free system for accurately measuring minority carrier lifetimes in semiconductor materials that possesses both linearity and sensitivity.
Further, the previously known devices and methods for measuring minority carrier lifetimes (xe2x80x9cxcfx84xe2x80x9d) were only capable of measuring minority carrier lifetimes in semiconductor materials having specific energy bandgaps, in direct or indirect bandgap semiconductor materials (i.e., typically not being able to measure both types of bandgap materials), and in semiconductor materials having a certain range of minority carrier lifetime values, i.e., generally relatively long lifetime values, e.g., at least about 40 nanoseconds, or relatively short, e.g., less than about 2 nanoseconds. For example, the presently practiced time-resolve photoluminescence (xe2x80x9cTRPLxe2x80x9d) method of measuring minority carrier lifetimes is only useful for measuring minority carrier lifetimes in direct bandgap semiconductors that have a bandgap greater than about 1.1 electronvolts (eV), which limits TRPL""s usefulness to a limited number of semiconductors that fit these criteria. Another known measurement method known as up-conversion TRPL is relatively effective for smaller bandgap semiconductor materials but only when the minority carrier lifetimes are very short, i.e., xcfx84 less than about 2 nanoseconds. Significantly, none of the known measuring devices provides a means for directly measuring minority carrier lifetimes in the range of about 2 to about 40 nanoseconds. Consequently, there remains a need for a single method and apparatus that is capable of measuring minority lifetimes in all types of semiconductor materials, i.e., in both direct bandgap materials and indirect bandgap materials which include silicon and germanium semiconductor materials, in semiconductors with a wide range of bandgap values (including materials with small bandgaps of less than 1.1 eV, such as GaAsN with between about 3 to 5 percent N, InAs, certain InGaAs alloys, InSb, and GaSb), and in semiconductors having relatively short minority carrier lifetimes, such as less than about 40 nanoseconds.
In addition to the disclosure of parent U.S. Pat. No. 5,929,652 referenced above, the inventors determined as discussed in the continuation-in-part application No. 09/283,738 and via additional analysis and empirical testing that additional variables and considerations must be taken into account to fully and accurately explain the linearity and sensitivity achieved by the disclosed apparatus. More specifically, in the parent U.S. Pat. No. 5,929,652 referenced above, the applicant had not recognized that the coil functions as an antenna in the disclosed apparatus, thereby creating a radiation resistance that alters and adds to the electrical impedance characteristics of the apparatus, particularly since the operation of the coil as an antenna may create a coupled impedance between the coil and the sample. Thus, the apparatus disclosed in the parent patent functions in a more complex manner than was originally believed, thereby potentially negating partially or completely the electrical circuit model discussed in the parent patent.
Accordingly, it is a general object of the present invention to provide apparatus and methods for measuring the minority carrier lifetimes and excess carrier recombination rates of semiconductor materials.
It is another general object of the present invention to provide apparatus and methods for accurately measuring minority carrier lifetimes and excess carrier recombination rates of semiconductor materials.
It is another object of the present invention to provide apparatus and methods for measuring minority carrier lifetimes and excess carrier recombination rates of semiconductor materials with consistent high sensitivity.
It is still another object of the present invention to provide apparatus and methods for measuring the minority carrier lifetimes and excess carrier recombination rates of semiconductor materials with consistent, accurate results for materials of different sizes, shapes, and other material properties.
It is a related object of the present invention to provide an apparatus and method for measuring the minority carrier lifetimes of both direct and indirect bandgap semiconductor materials.
It is another related object of the present invention to provide an apparatus and method for measuring the minority carrier lifetimes of semiconductor materials having a relatively short minority carrier lifetime and/or having a relatively small bandgap.
It is yet another object of the present invention to provide apparatus and methods for the contactless measurement of minority carrier lifetimes and excess carrier recombination rates of semiconductor materials.
Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, an apparatus in accordance with the present invention includes a coil capable of generating electromagnetic radiation when an electric current flows through the coil; a positioner capable of positioning the sample in proximity to the coil; a light source; a bridge circuit having four nodes connected by four branches, three of said four branches including resistive elements and the remaining branch including a capacitive element in electrical parallel with a connection to the coil; and an oscillator connected to two of the nodes of said bridge circuit such that the oscillator can apply a voltage signal to the two nodes of the bridge circuit.
To further achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, a method in accordance with the present invention for determining carrier lifetime or recombination rate of a semiconductor sample includes applying an input voltage signal having a frequency to two nodes of a bridge circuit having four branches, wherein three branches of the bridge circuit include resistive elements and a fourth branch of the bridge circuit includes a primary capacitive element in parallel with a connection to a coil capable of generating electromagnetic radiation when an electric current flows through the coil; positioning the sample in proximity to the coil; balancing the bridge circuit by adjusting position of the sample relative to the coil; illuminating the sample for a finite period of time; and measuring an output voltage signal at two nodes of the bridge circuit which are different from the two nodes of the bridge circuit to which the input voltage signal is applied.
Also to achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, a method in accordance with the present invention for measuring carrier lifetime or recombination rate of a semiconductor material sample, includes applying an input voltage signal having a frequency to two nodes of a bridge circuit having four branches, wherein three branches of the bridge circuit include resistive elements and a fourth branch of the bridge circuit includes a capacitive element in parallel with a connection to a coil; conducting an electric current through the coil; positioning the semiconductor material in proximity to the coil; calibrating the frequency of the input voltage signal; balancing the bridge circuit by adjusting mutual inductance and/or coupled impedance between sample semiconductor material and the coil; illuminating the semiconductor material for a finite period of time; and measuring an output voltage signal at two nodes of the bridge circuit which are different from the two nodes of the bridge circuit to which said input voltage signal is applied.
Also to achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, an apparatus in accordance with the present invention includes a bridge circuit having four nodes connected by four branches, three of which branches have resistive components and a fourth of which branches has a capacitive component; a coil capable of generating electromagnetic radiation when electric current flows through the coil positioned at a distance spaced apart from the semiconductor material in an adjustable manner such that the distance is variable, the coil being connected electrically to the fourth branch of the bridge circuit in parallel with the capacitance component; an oscillator connected to two of the nodes of the bridge circuit in such a manner that the oscillator imparts a voltage signal across the two nodes; a light source positioned to illuminate the semiconductor material with a pulse of light; and a voltage detector connected electrically across two of the nodes of the bridge circuit that are not the same nodes to which the oscillator is connected.
Also to achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, a method in accordance with the present invention for determining carrier lifetime of a sample of semiconductor material includes providing a circuit capable of creating an output signal indicative of the sample""s conductivity when the sample is illuminated and of eliminating any portion of the output signal that is indicative of the sample""s dark conductivity; electromagnetically coupling the circuit and the sample; adjusting mutual inductance and/or coupled impedance between the sample and the circuit such that the output signal is eliminated; illuminating the sample for a finite period of time; and measuring the output signal.
To further achieve the above and other objectives, an alternative measuring apparatus is provided that is configured with components to provide higher efficiency radio-frequency coupling between a semiconductor sample and an included antenna and to provide reduced system response times. These aspects of the measuring apparatus enable the measuring apparatus to measure minority carrier lifetimes in semiconductor samples having a short lifetime (i.e., less than about 40 nanoseconds), having any sized bandgap (including less than about 1.1 eV), and/or measuring minority carrier lifetimes in both direct and indirect bandgap materials. To increase radio-coupling efficiency, the measuring apparatus includes a signal generator that is adapted to operate at high frequencies, such as about 400 MHz and more preferably, above about 900 MHz. The signal generator transmits a sinusoidal waveform signal that is split by the measuring apparatus into a reference signal and a sample signal. The sample signal is transmitted by a high frequency, low reactance antenna to a coupled semiconductor sample which reflects varying intensity sample-coupled-photoconductivity signals back to the antenna in response to light pulses striking the sample from a laser. The measuring apparatus uses measured changes in the sample-coupled-photoconductivity signal intensity or power to determine the minority carrier lifetime of the sample.
According to an important feature of the measuring apparatus, the sample signal enters a sample branch circuit in which impedance is matched to improve system response, i.e., to reduce the time for power to flow through the included components, thereby allowing the measuring apparatus to measure minority carrier lifetimes in the 2 to 40 nanosecond range which previously had not been successfully accomplished. As an example of impedance xe2x80x9cmatching,xe2x80x9d in a 50 ohm system, an antenna having 50 ohm real resistance would have optimal transient response. As may be understood by those skilled in the art, large antenna reactance and small resistance provides a large quality factor, Q, which results in good signal sensitivity but longer transient times. Conversely, reducing antenna reactance may lower the quality factor, Q, but provides faster transient times and faster system response. In this regard, higher operating frequencies provides room for increased bandwidth, which is necessary to resolve shorter transients corresponding with faster system response. With these effects in mind and to lower impedance, the antenna used to transmit the sample signal to the sample and to receive the reflected power from the sample (i.e., the sample-coupled-photoconductivity signal) is preferably selected to have a relatively low antenna reactance, as well as being operable at higher operating frequency. In one preferred embodiment, the antenna is a hybrid waveguide/aperture antenna with a centrally positioned transmission line and a ground flange adjacent the transmission line. The antenna is selectively positioned, with a positioner or other device, a coupling distance from the sample. To further control or lower impedance, a cable with a relatively short length, i.e., preferably less than 1 wavelength and more preferably less than about xc2xc wavelength, is used to transmit the sample signal from a tuning capacitor or other matching element to the antenna. In other embodiments, the antenna and coupled sample is impedance-matched to the system (for example, 50 ohms) by using lumped reactive element (e.g., a capacitor) matching, series or shunt transmission line stub matching, or quarter-wave transformer matching. Additionally, in the sample branch circuit, shorter interconnecting cables are used to reduce loss and increase signal sensitivity, but preferably are a certain, known fraction of the sample signal wavelength for furthering impedance matching.
According to another important feature of the measuring apparatus, the reference signal and the signal in the sample branch circuit, i.e., the sample-coupled-photoconductivity signal, are phase matched which acts to operatively link the signal generator and the signal detection portion of the measuring apparatus, i.e., the antenna, the tuning capacitor, and other components. Phase matching of these signals enables the measuring apparatus to measure very small changes in the reflected power or sample-coupled-photoconductivity signal, thereby significantly enhancing the sensitivity of the measuring apparatus in measuring minority carrier lifetimes. Phase matching can be accomplished in several ways, including operating a phase shifter included in the reference signal branch of the measuring apparatus and operating a mixer used to receive the reference signal and the sample-coupled-photoconductivity signal so as to maximize the output signal from the mixer.